Semiconductor integrated circuit with PLL circuit

ABSTRACT

In a PLL circuit including an oscillator, a phase comparator, a charge pump circuit, and a loop filter, without providing a plurality of capacitative elements, that is, without increasing the occupied area so much, the characteristics of the PLL circuit can be adjusted according to manufacture variations in a resistive element and a capacitative element, and a loop filter can be formed on a chip. A resistive element and a capacitative element of a loop filter are formed on a semiconductor chip. As the resistive element, a plurality of elements having different resistance values are provided and switched by a switch, thereby enabling the resistance value to be adjusted. Current in a charge pump circuit is also made adjustable, and the current of the charge pump circuit is adjusted according to switching among the resistance values of the resistive elements.

This application is a continuation application of U.S. application Ser.No. 11/178,565, filed Jul. 12, 2005, now allowed, the entirety of whichis incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationNo 2004-205452 filed on Jul. 13, 2004, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a PLL (Phase Locked Loop) circuitincluding a voltage-controlled oscillator (VCO) and, more particularly,to a technique effective when applied to the case where a filter on aPLL loop is provided in a semiconductor chip. The invention relates to,for example, a technique effective for use in a PLL circuit which isprovided in a semiconductor integrated circuit for high frequencies formodulating or up-converting a transmission signal in a radiocommunication device such as a cellular phone.

A radio communication device (mobile communication device) typified by acellular phone is generally constructed by: a semiconductor integratedcircuit (generally, called RF IC) having the function of up-convertingand modulating a transmission signal and down-converting anddemodulating a reception signal; a semiconductor integrated circuit(baseband IC) having the function of converting transmission data to Iand Q signals and reconstructing reception data from demodulated I and Qsignals; an electronic part called a power module including ahigh-frequency power amplifier, a bias circuit for the power amplifier,and an impedance matching circuit; and an electronic part called afront-end module including a transmission/reception switching circuit, alow-pass filter, and an impedance matching circuit.

In recent years, in order to reduce the size and cost of a radiocommunication device by decreasing the number of parts, efforts arebeing made to provide circuits as many as possible in one or a fewsemiconductor integrated circuits. As one of them, an attempt is made toform resistive elements and capacitative elements as components of aloop filter provided on a PLL in an RF IC onto a semiconductor chip.

As it is known, however, in the present semiconductor integrated circuitmanufacturing technique, variations in the resistive elements andcapacitative elements formed on a semiconductor chip are large, andthere is a problem such that the characteristics of the loop filter aredeviated from desired characteristics. Consequently, in a conventionalRF IC, a loop filter is generally constructed by external resistiveelements and external capacitative elements, and it is one of factorsdisturbing miniaturization. An invention has been proposed in which, forminiaturization of an oscillation circuit, resistive and capacitativeelements as components of a loop filter are formed on a semiconductorchip and an element to be used is switched by a switch so that thecharacteristics of the filter can be adjusted (Japanese UnexaminedPatent Publication No. Hei 09(1997)-331251).

SUMMARY OF THE INVENTION

In the conventional technique of preliminarily forming a plurality ofresistive elements and a capacitative element on a semiconductor chipand adjusting the characteristics of a filter, particularly, the size ofthe capacitative element as a component of a loop filter is large, sothat the area occupied by the filter is large. It causes a problem suchthat the chip size increases and the unit price of a chip rises.

There is a method of providing, as the capacitative element, a pluralityof capacitative elements corresponding to the difference from a basiccapacitative element, and additionally connecting a capacitative elementcorresponding to the difference in accordance with a manufacturevariation to obtain a desired capacitance value, thereby minimizingincrease in the occupied area. In the case of connecting a capacitativeelement, however, parasitic capacitance of a wire and a switch for theconnection cannot be ignored. Consequently, in the method ofadditionally connecting a capacitative element corresponding to thedifference, when the number of capacitative elements to be connected isdetermined in consideration of only the area ratio of the capacitativeelements, high-precision adjustment cannot be performed. On the otherhand, when the area of the capacitative elements corresponding to thedifference is preliminarily determined in consideration of the parasiticcapacitance of a wire and a switch for connection, the parasiticcapacitance has to be estimated. Consequently, designing becomes verytroublesome and the capacitative element itself also has a manufacturevariation. There is a problem such that high-precision adjustment cannotbe made as a result.

An object of the present invention is to provide a circuit techniquecapable of adjusting the characteristics of a PLL circuit including anoscillator, a phase comparator, a charge pump circuit, and a loop filterin accordance with manufacture variations in a resistive element and acapacitative element without providing a plurality of capacitativeelements, that is, without increasing an occupied area so much and,capable of forming a loop filter on a chip.

Another object of the invention is to provide a semiconductor integratedcircuit with a PLL circuit in which, even when a loop filter isconstructed by elements on a chip, variations in characteristics due tovariations in the elements can be corrected, and the yield does notdeteriorate.

Further another object of the invention is to provide a semiconductorintegrated circuit with a PLL circuit, in which the characteristics of abuilt-in loop filter can be adjusted without increasing a manufacturingstep and, even when the loop filter is formed on a chip, the manufacturecost is not so increased.

The above and other objects of the invention and novel features willbecome apparent from the description of the specification and theappended drawings.

An outline of representative ones of inventions disclosed in theapplication will be described as follows.

The invention provides a semiconductor integrated circuit with a PLLcircuit, including an oscillator, a phase comparator, a charge pumpcircuit, and a loop filter, for controlling the oscillator with avoltage obtained by smoothing an output of the charge pump circuit bythe loop filter. A resistive element and a capacitative element ascomponents of the loop filter are formed on a semiconductor chip. Theresistive element is constructed by a plurality of elements havingdifferent resistance values which are switched by a switch, therebyenabling the resistance value to be adjusted. An output current of thecharge pump circuit is also made adjustable according to the switchamong the resistance values of the resistive elements.

Preferably, the following circuits are provided on the samesemiconductor chip as that of the PLL circuit. The circuits are acurrent circuit (hereinbelow, called a current monitor circuit)including a resistive element and a capacitative element which matchwell with the resistive element and the capacitative element of the loopfilter and having a configuration similar to that of the charge pump ofthe PLL circuit, a current-voltage converter for converting a monitorcurrent generated by the current monitor circuit into a voltage, and acorrecting circuit for measuring time elapsed before the convertedvoltage reaches a predetermined level, estimating the product of aresistance value and a capacitance value of the loop filter on the basisof the measured time, and adjusting the resistance value of theresistive element of the loop filter and the current in the charge pumpcircuit in accordance with the estimated product.

With the means, in the case where the capacitance value of thecapacitative element as a component of the loop filter varies, byadjusting the current in the charge pump circuit and the resistiveelement in the filter, the characteristic (open loop gaincharacteristic) of the PLL circuit can be changed close to a desiredcharacteristic. Thus, the characteristic can be corrected withoutproviding a plurality of capacitative elements for adjustment. Since thedesired characteristic can be obtained by correction even when theresistive element and the capacitative element as components of the loopfilter are formed on a chip, the yield of the semiconductor integratedcircuit with the PLL circuit does not deteriorate.

Further, in the case where the resistance value of the resistive elementas a component of the loop filter varies, the resistance value isswitched by a switch in accordance with the variation, thereby adjustingthe resistance value to make the characteristic close to the desiredfilter characteristic. Moreover, the resistance value is switched byusing a control signal of adjusting the current of the charge pumpcircuit, so that the scale of the correcting circuit for generating acontrol signal for adjustment can be decreased, and increase in the chipsize can be suppressed. By providing the correcting circuit on the samesemiconductor chip as that of the PLL circuit, it becomes unnecessary tomeasure the resistance value of the resistive element and thecapacitance value of the capacitative element in the loop filter by aprobe test or the like and correct variations by trimming using a fuseand the like. Consequently, increase in the cost can be avoided.

According to the invention, also in the case where the resistive elementand/or the capacitative element as components of the filter varies, byadjusting both of the resistive element in the filter and the resistiveelement which determines the current in the charge pump circuit, thecharacteristic of the PLL circuit can be corrected so as to be close tothe desired characteristic.

Effects obtained by the representative one of the inventions disclosedin the application will be briefly described as follows.

In a PLL circuit including an oscillator, a phase comparator, a chargepump circuit, and a loop filter, without providing a plurality ofcapacitative elements, that is, without increasing an occupied area somuch, the characteristic of the PLL circuit can be adjusted inaccordance with manufacture variations in the resistive element and thecapacitative element so that a loop filter can be formed on a chip.

Even when a loop filter is constructed by elements on a chip, variationsin characteristics due to variations in the elements can be corrected,and the yield does not deteriorate. In addition, without increasing amanufacturing step, the characteristics of a built-in loop filter can beadjusted. Consequently, even when the loop filter is formed on a chip, asemiconductor integrated circuit with the PLL circuit whose manufacturecost is not so increased can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of the configuration of aPLL circuit to which the invention is suitably applied.

FIG. 2 is a characteristic diagram showing a frequency characteristic ofa loop filter in FIG. 1.

FIG. 3 is a circuit diagram showing a concrete example of acharacteristic correcting circuit for correcting characteristics of acharge pump and a loop filter in the PLL circuit of the embodiment.

FIG. 4 is a graph showing a state of variations in required time Tccaused by variations in resistive elements and capacitative elements.

FIG. 5 is a circuit diagram showing a configuration example of athird-order loop filter.

FIG. 6 is a block diagram showing an embodiment of applying a PLLcircuit of the invention to an RF IC having the function ofmodulating/demodulating a transmission/reception signal and aconfiguration example of a radio communication system.

FIG. 7 is a block diagram showing another embodiment of applying the PLLcircuit of the invention to an RF IC having the function ofmodulating/demodulating a transmission/reception signal and aconfiguration example of a radio communication system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinbelow with reference to the drawings.

FIG. 1 shows a configuration example of a PLL circuit to which theinvention is preferably applied.

A PLL circuit of the embodiment has a voltage-controlled oscillator(VCO) 11, a variable frequency divider 12 for frequency-dividing anoscillation signal φ0 of the VCO 11 to 1/N on the basis of a frequencydivision ratio N which is set from the outside, a fixed frequencydivider 14 for frequency-dividing an oscillation signal φr from areference oscillator (DCXO) 13 for generating a reference oscillationsignal φr such as 26 MHz, a phase comparator 15 for detecting the phasedifference between signals φ41 and φr′ subjected to frequency divisionin the variable frequency divider 12 and the fixed frequency divider 14,respectively, a charge pump 16 for generating a charge current ordischarge current according to the detected phase difference, and a loopfilter 17 for smoothing an output of the charge pump 16. The voltagesmoothed by the loop filter 17 is fed back as a control voltage Vt tothe VCO 11 to control the oscillation frequency of the VCO 11.

As the loop filter 17, although not limited, in the embodiment, asecond-order lag-lead filter having a capacitative element C1 connectedin series between an output terminal of the charge pump 16 and the earthpoint, and a capacitative element C2 and a resistive element R2connected in series and provided in parallel with the capacitativeelement C1 is used. The charge pump 16 has a current source CS1 forcharge-up and a current source CS2 for charge-down. When the phase ofthe signal φ1 obtained by frequency division of the variable frequencydivider 12 lags behind the phase of the reference signal φr′, current ofthe current source CS1 for charge-up is increased so that the smoothedvoltage Vt of the loop filter 17 becomes high and the oscillationfrequency of the VCO 11 becomes high. On the other hand, when the phaseof the signal φ1 obtained by frequency division of the variablefrequency divider 12 leads the phase of the reference signal φr′,current of the current source CS2 for charge-down is increased so thatthe smoothed voltage Vt of the loop filter 17 becomes low and theoscillation frequency of the VCO 11 becomes low.

FIG. 2 shows a frequency characteristic of an open loop of the PLLcircuit in FIG. 1 when the horizontal axis indicates frequency inlogarithm scale. The graph has the zero point at frequency f1, and hasthe pole at frequency f2. To make the characteristics of the loop filterconstant, the open loop gain characteristic has to be made constant.When the current of the charge pump 16 is Ipd, the gain of the VCO 11 isKv, and the frequency division ratio of the variable frequency divider12 is N, transfer function G0(s) of the open loop gain is expressed bythe following equation. $\begin{matrix}{{G\quad 0(s)} = {\frac{{Ipd} \cdot {kv}}{\left( {{C\quad 1} + {C\quad 2}} \right) \cdot N} \cdot \frac{1 + \frac{s}{\omega\quad 1}}{s^{2} \cdot \left( {1 + \frac{s}{{\omega\quad 2}\quad}} \right)}}} & (1)\end{matrix}$

Since the gain Kv of the VCO 11 and the frequency division ratio N ofthe variable frequency divider 12 can be regarded as constant, it isunderstood that by making Ipd/(C1+C2), ω1, and ω2 constant, the transferfunction G0(s) can be made constant. ω1 and ω2 are values correspondingto the frequencies f1 and f2 of the zero point and the pole,respectively, in the open loop gain characteristic of FIG. 2, and areexpressed as f1=ω1/2π and f2=ω2/2π, respectively.

The zero point f1 and the pole f2 are expressed by the followingequations.f1=1/{2π·C2·R2}  (2)f2=(C1+C2)/(2π·C1·C2·R2)  (3)

When it is assumed that C2 is sufficiently larger than C1, Equation (2)can be modified as the following equation.f2≈½π·C1·R2  (4)

It is understood that, by making the products C2·R2 and C1·R2 ofresistance and capacitance constant by Equations (2) and (4), thepositions of the zero point f1 and the pole f2 can be maintained almostconstant.

FIG. 3 shows a concrete circuit example of a characteristic correctingcircuit 18 for correcting characteristics of the charge pump 16 and theloop filter 17 in the PLL circuit of the embodiment developed from theabove-described viewpoint.

The loop filter 17 of the embodiment has a configuration such that theresistive element R2 of the loop filter constructed by the capacitativeelements C1 and C2 and the resistive element R2 shown in FIG. 1 isreplaced with three resistive elements R1, R2, and R3 having differentresistance values and formed in parallel and switch elements SW1, SW2,and SW3 connected to the resistive elements R1, R2, and R3 in series,respectively. One of the switch elements SW1, SW2, and SW3 isselectively turned on by control signals CS1, CS2, and CS3 from thecharacteristic correcting circuit 18, thereby enabling thecharacteristic of the filter to be corrected. For example, when theresistance value of the resistive element R2 is 1, the resistance valueof the resistive element R1 is 0.9 time, and that of the resistiveelement R3 is 1.1 times.

The charge pump 16 has a current circuit 16A for passing charge-upcurrent Ipu to the loop filter 17 in accordance with a signal UPindicative of phase lag from the phase comparator 15, and a currentcircuit 16B for passing charge-down current Ipd for charging down theloop filter 17 in accordance with a signal DOWN indicative of phase leadfrom the phase comparator 15.

The current circuit 16A is constructed by a switch SW10 which is turnedon/off according to the signal UP indicative of phase lag from the phasecomparator 15, a resistor R10 and bipolar transistors Q11 and Q12 whichare connected in series with the switch SW10, a bipolar transistor Q13whose base terminal is connected to a connection node between theresistor R10 and the transistor Q11, three resistive elements R11, R12,and R13 connected in parallel between the collector of the bipolartransistor Q13 and the earth point, switch elements SW11, SW12, and SW13connected in series with the resistive elements R11, R12, and R13,respectively, a MOS transistor Q14 connected between the emitter of thetransistor Q13 and a power source voltage terminal Vcc, and a MOStransistor Q15 whose gate is commonly connected with the gate of the MOStransistor Q14.

The transistors Q11 and Q12 each having a base and a collector coupledto each other act as a diode and apply, as a bias voltage, a potentialtwice as large as a base-emitter voltage VBE when the switch SW10 is inan ON state to the base of the transistor Q13, thereby passing collectorcurrent to the transistor Q13. The gate and drain of the MOS transistorQ14 are coupled to each other, and the MOS transistor Q14 functions as acurrent-voltage converting device. The gates of the transistors Q14 andQ15 are commonly connected, so that the transistors Q14 and Q15construct a current mirror circuit. A current proportional to a currentflowing in the transistor Q14 is passed to the transistor Q15 and isoutput as the charge-up current Ipu. In the embodiment, the transistorsQ14 and Q15 are formed so as to have the same size, so that currentwhose magnitude is the same as that of the current flowing in thetransistor Q14 is passed to the transistor Q15.

One of the switch elements SW11, SW12, and SW13 is selectively turned onby the control signals CS1, CS2, and CS3 from the characteristiccorrecting circuit 18, thereby enabling the collector current of thetransistor Q13 and the charge-up current Ipu to be changed. For example,when the resistance value of the resistive element R12 is 1, theresistance value of the resistive element R11 is 0.9 time, and that ofthe resistive element R13 is 1.1 times.

The current circuit 16B has a configuration similar to that of thecurrent circuit 16A. The different point is that, since the currentcircuit 16B is a current circuit for discharging, a diode-connected MOStransistor Q26 connected in series with a MOS transistor Q25corresponding to the MOS transistor Q15 in the current circuit 16A, anda MOS transistor Q27 whose gate is commonly connected with the gate ofthe MOS transistor Q26 to construct a current mirror circuit areprovided, and by sending back the current of the MOS transistor Q26 bythe current mirror Q26, the discharge current Ipd is generated.

The characteristic correcting circuit 18 is constructed by: a constantvoltage circuit 181 such as a band gap reference circuit for generatingconstant voltages V0 and V1 having no power source voltage dependencyand no temperature dependency; a current monitor circuit 182 having MOStransistors Q31 and Q32 having gate terminals to which the constantvoltage V0 generated by the constant voltage circuit 181 is applied andwhich are commonly connected to construct a current mirror circuit; acomparator 183 for comparing a voltage obtained by smoothing a monitorcurrent of the current monitor circuit 182 by capacitance C0 with theconstant voltage V1 generated by the constant voltage circuit 181; and acontrol circuit 184 having a timer TMR for measuring time that elapsesbefore an output of the comparator 183 is inverted, and outputting thecontrol signals CS1 to CS3 for turning on/off the switch elements SW11to SW13 and SW21 to SW23 in the charge pump 16 and the switch elementsSW1 to SW3 in the loop filter 17 in accordance with the measured time.The timer TMR can be constructed by a counter for counting oscillationsignals φr from the reference oscillator (DCXO) 13.

The current monitor circuit 182 has a resistor R0 connected in serieswith the MOS transistor Q31 for the current mirror, generates a monitorcurrent I0 in which variations in the resistor R0 are reflected, and hasa capacitative element C0 charged with the monitor current I0, and aswitch element SW0 for resetting the capacitative element C0 bydischarging. When charging of the capacitative element C0 is reset andthe reset is cancelled, the capacitative element C0 is charged with thecurrent I0 and its voltage V0 gradually increases. When the voltage V0reaches the constant voltage V1, an output of the comparator 183 isinverted. Therefore, time Tc required for an output of the comparator183 to be inverted after the cancel of the reset is proportional to thecapacitance value of the capacitative element C0 and is inverselyproportional to the current value of the monitor current I0, that is,Tc∝C0/I0.

The monitor current I0 is current in which variations of the resistiveelement R0 are reflected as described above and the resistive element R0is constructed by a resistive element which matches well with theresistive elements R11 to R13 and R21 to R23 in the charge pump 16. Withthe configuration, the monitor current I0 can be formed as currenthaving variations similar to those of the output currents Ipu and Ipd ofthe charge pump 16 irrespective of manufacturing variations in theresistive elements. In the embodiment, a capacitative element whichmatches well with the capacitative elements C1 and C2 as components ofthe loop filter 17 is used as the capacitative element C0 in the currentmonitor circuit 182, and a resistive element which matches well with theresistive element R2 as a component of the loop filter 17 is used as theresistive elements R11 to R13 and R21 to R23 in the charge pump 16.

Required time Tc for the voltage V0 reaches V1 is measured by the timerin the control circuit 184. When Tc is larger than a target value T0,the output currents Ipu and Ipd of the charge pump 16 are increased.When Tc is smaller than the target value T0, the output currents Ipu andIpd of the charge pump 16 are decreased. In such a manner, Ipd/(C1+C2)expressed by Equation (1) can be made constant. To increase the outputcurrents Ipu and Ipd of the charge pump 16, it is sufficient to selectsmall one of resistance values of the resistive elements R11 to R13 andR21 to R23. To decrease the output currents Ipu and Ipd of the chargepump 16, it is sufficient to select large one of resistance values ofthe resistive elements R11 to R13 and R21 to R23.

The control circuit 184 in FIG. 3 is constructed to generate the controlsignals CS1 to CS3 for performing selecting operation as describedabove. The control circuit 184 generates a reset signal RST for turningon the switch element SW0 to reset charging of the capacitive elementC0, and has a register REG for holding the states of the control signalsCS1 to CS3 determined on the basis of the required time Tc that elapsesbefore the output of the comparator 183 is inverted.

Further, in the embodiment, the resistive elements R1 to R3 in the loopfilter 17 are also selected by the control signals CS1 to CS3 whichselect the resistive elements R11 to R13 and R21 to R23 in the chargepump 16. Consequently, not only variations in the current in the chargepump but also variations in the characteristics of the loop filter canbe corrected, and ω1 and ω2 in Equation (1) can be made constant. As aresult, by correcting the output currents Ipu and Ipd in the charge pump16, not only Ipd/(C1+C2) but also the transfer function G0(s) can bemade constant, and the open loop gain characteristic of the PLL circuitcan be made constant. In the embodiment, by providing the correctingcircuit 18 as a circuit common to the charge pump 16 and the loop filter17, enlargement of the circuit scale can be suppressed.

The reason why ω1 and ω2 in Equation (1) can be made constant in theembodiment will now be described. As described above, the currentmonitor circuit 182 is provided to detect the required time Tc whichelapses before the capacitative element C0 is charged with the monitorcurrent I0 and the voltage Vc reaches the predetermined voltage V1, andTc is proportional to the capacitance value of the capacitative elementC0 and is inversely proportional to the current value of the monitorcurrent I0, so that Tc∝C0/I0 is satisfied. Since the current value ofthe monitor current I0 is inversely proportional to the resistance valueof the resistive element R0, Tc∝R0·C0 is satisfied, and Tc isproportional to the product between resistance and capacitance.Therefore, when the required time Tc is measured and the resistancevalue of the loop filter is changed so that the required time Tc becomesconstant, even if the resistance and capacitance varies, the productbetween resistance and capacitance is maintained constant. As describedby using Equations (2) and (4), the positions of the zero point f1 andthe pole f2 can be made almost constant. As a result, the open loop gaincharacteristic of the PLL circuit can be made constant.

FIG. 4 shows a state of variations in the required time Tc due tovariations in the resistive and capacitative elements, and Table 1 showsthe relations among the control signals CS1 to CS3 generated incorrespondence with the variations in the required time Tc and theresistive elements selected by the control signals CS1 to CS3.

In FIG. 4, the solid line A shows the case where there is no variation.The broken line B shows the case where resistance and/or capacitancevaries to the increase side and the required time Tc becomes long. Thelong and short dash line D indicates the case where resistance and/orcapacitance varies to the decrease side and the required time Tc becomesshort. The required time Tc becomes long when resistance varies to theincrease side, and becomes short when resistance varies to the decreaseside for the reason that the monitor current I0 decreases when theresistance R0 increases in the monitor circuit 182 in FIG. 3, andincreases when the resistance R0 decreases. TABLE 1 Product ofcapacitance Filter Charge pump measurement and selected selected resultof Tc resistance CS1 CS2 CS3 resistance resistance T0  ±0% L H L R2 R12,R22 T1(0.9 × T0) −10% L L H R3(1.1R2) R13, R23(1.1 times) T2(1.1 × T0)+10% H L L R1(0.9R2) R11, R21 (0.9 time)

As understood from Table 1, when the required time Tc becomes shorter,the switches SW3, SW13, and SW23 are turned on to select the resistiveelements R3, R13, and R23 having large resistance values. When therequired time Tc becomes longer, the switches SW1, SW11, and SW21 areturned on to select the resistive elements R1, R11, and R21 having smallresistance values.

In the description of FIG. 3, as the resistive element R0 in the monitorcircuit 182 and the resistive elements R11 to R13 and R21 to R23 in thecharge pump 16, resistive elements which matches well with the resistiveelement R2 as a component of the loop filter 17 are used. As thecapacitative element C0 in the monitor circuit 182, a capacitativeelement which matches well with the capacitative elements C1 and C2 ascomponents of the loop filter 17 is used. In other words, elements areformed of the same material in the same direction in the same step.Consequently, when an element as a component of the loop filter 17varies, an element as a component of the monitor circuit 182 and anelement as a component of the charge pump 16 similarly vary. Thus,accurate characteristic correction can be made. From the viewpoint ofsetting the degrees of variations to be the same as much as possible, itis desirable to form elements so as to be close to each other on asemiconductor chip.

In the embodiment, as each of the resistive element in the charge pump16 and the resistive element in the loop filter 17, three resistiveelements having different resistance values are provided and any one ofthem is selected. The number of resistive elements is not limited tothree but may be four or more. The larger the number of resistiveelements prepared is, the characteristic can be corrected with higherprecision. In the embodiment of FIG. 3, the correcting circuit 18 isprovided as a circuit common to the charge pump 16 and the loop filter17. Alternately, correcting circuits can be separately provided for thecharge pump 16 and the loop filter 17. Variation in current in thecharge pump and variation in resistance of the loop filter can becorrected separately, so that optimum correction can be made.

In the correcting circuit 18 of the foregoing embodiment, the constantvoltage V0 from the constant voltage circuit 181 is applied to the gateterminals of the MOS transistors Q31 and Q32. It is also possible toprovide a bipolar transistor corresponding to the transistor Q13 in thecharge pump of FIG. 3 between the MOS transistor Q31 and the resistor R0in the correcting circuit 18, apply the constant voltage V0 from theconstant voltage circuit 181 to the base terminal of the bipolartransistor, and pass current proportional to the resistance value of theresistor R0.

Although it is described in the foregoing embodiment that the chargepump 16 is constructed as a circuit separate from the phase comparator15, depending on a circuit form employed, there is a case such that theoutput stage of the phase comparator 15 has the function ofpassing/receiving current according to the phase difference. It can bealso regarded that the charge pumps 16A and 16B in FIG. 3 construct theoutput stage of the phase comparator 15. Therefore, inventions have tobe substantially compared with each other without sticking to the namesof circuits.

Although the case of applying the invention to the PLL circuit using thesecond-order loop filter has been described in the foregoing embodiment,the invention can be also applied to a PLL circuit using a third-orderloop filter. FIG. 5 shows a configuration example of a third-order loopfilter. The third-order loop filter has a configuration obtained byadding a resistor R4 and a capacitor C3 to the second-order loop filtershown in FIG. 3.

In the third-order loop filter of FIG. 5, like the second-order loopfilter shown in FIG. 3, the resistor connected in series with thecapacitor C2 is constructed by the plurality of resistors R1 to R3having resistance values different from each other. A plurality ofresistors are not provided as the resistor R4 since the resistor R4gives the frequency pole much higher than f2 in FIG. 2 and hardly exertsan influence to f1 and f2. In the case of adjusting the open loop gaincharacteristic with higher precision, preferably, the resistor R4 isreplaced with a plurality of resistors connected in parallel like theresistors R1 to R3, and the plurality of resistors are switched.

Next, an embodiment of applying the PLL circuit according to theinvention to an RF-IC having the function of modulating/demodulatingtransmission/reception signals and a configuration example of a radiocommunication system will be described by using FIGS. 6 and 7. Atransmission system in each of FIGS. 6 and 7 is constructed by an offsetPLL system.

As shown in FIG. 6, the radio communication system of the embodimentincludes an antenna 400 for transmitting/receiving signal electricwaves, a switch 410 for switching transmission/reception, band-passfilters 420 a to 420 d taking the form of, for example, SAW filters thateliminates unnecessary waves from a reception signal, a high-frequencypower amplifier (power module) 430 for amplifying a transmission signal,a radio-frequency IC 200 for demodulating a reception signal andmodulating a transmission signal, and a baseband circuit 300 forconverting transmission data to I and Q signals and controlling the RFIC 200. In the embodiment, the RF IC 200 and the baseband circuit 300are constructed as semiconductor integrated circuits on differentsemiconductor chips.

Although not limited, the RF IC 200 can modulate/demodulate signals infour frequency bands by communication methods of GSM850, GSM900,DCS1800, and PCS1900. Accordingly, as the band pass filters, the filter420 a passing a reception signal in the frequency band of GSM850, thefilter 420 b passing a reception signal in the frequency band of GSM900,the filter 420 c passing a reception signal in the frequency band ofDCS1800, and the filter 420 d passing a reception signal in thefrequency band of PCS1900 are provided.

The RF IC 200 of the embodiment is constructed roughly by a receptioncircuit RXC, a transmission circuit TXC, and a control system includingcircuits common to the transmission and reception systems, such as othercontrol circuits and clock generating circuits.

The reception circuit RXC includes: low-noise amplifiers 210 a to 210 dfor amplifying reception signals in the frequency bands of GSM850,GSM900, DCS1800, and PCS1900; a frequency-division phase-shift circuit211 for frequency-dividing a local oscillation signal φRF generated by aradio frequency oscillator (RFVCO) 250 to generate orthogonal signalswhose phases are shifted from each other by 90°; mixers 212 a and 212 bfor mixing the orthogonal signals generated by the frequency-divisionphase-shift circuit 211 with the reception signals amplified by the lownoise amplifiers 210 a to 210 d, thereby demodulating anddown-converting I and Q signals; high gain amplifiers 220A and 220Bcommon to the frequency bands, which amplify the demodulated I and Qsignals and output the resultant signals to the baseband circuit 300;and an offset cancel circuit 213 for canceling an input DC offset ofamplifiers in the high gain amplifiers 220A and 220B.

The high gain amplifier 220A has a configuration in which a plurality oflow pass filters LPF11, LPF12, LPF13, and LPF14 and gain controlamplifiers PGA11, PGA12, and PGA13 are alternately connected in series,and an amplifier AMP1 is connected in the final stage. The high gainamplifier 220A amplifies the demodulated I signal to a predeterminedamplification level while removing unnecessary waves. The high gainamplifier 220B similarly has a configuration in which a plurality of lowpass filters LPF21, LPF22, LPF23, and LPF24 and gain control amplifiersPGA21, PGA22, and PGA23 are alternately connected in series, and anamplifier AMP2 is connected in the final stage. The high gain amplifier220B amplifies the demodulated Q signal to a predetermined amplificationlevel.

The offset cancel circuit 213 includes: A/D converters (ADC) provided incorrespondence with the gain control amplifiers PGA11 to PGA23 andconverting an output potential difference in a state where inputterminals are short-circuited into a digital signal; D/A converters(DAC) each generating an input offset voltage which sets a DC offset ofan output of corresponding one of the gain control amplifiers PGA11 toPGA23 to zero and applying the input offset voltage for a differentialinput; and a control circuit for controlling the A/D converters (ADC)and the D/A converters (DAC) to perform an offset canceling operation.

The transmission circuit TXC includes: an oscillator (IFVCO) 230 forgenerating an oscillation signal φIF of an intermediate frequency suchas 640 MHz; a frequency-division phase-shift circuit 232 forfrequency-dividing the oscillation signal φIF generated by theoscillator 230 to generate orthogonal signals whose phases are shiftedfrom each other by 90°; orthogonal modulation circuits 233 a and 233 bwhich are mixers for modulating the generated orthogonal signals by theI and Q signals supplied from the baseband circuit 300; an adder 234 foradding modulated signals; an oscillator (TXVCO) 240 for transmission forgenerating a transmission signal φTX of a predetermined frequency; anoffset mixer 235 for mixing a feedback signal obtained by extracting thetransmission signal φTX output from the oscillator 240 for transmissionby a coupler or the like and a signal φRF′ obtained byfrequency-dividing the oscillation signal φRF generated by theradio-frequency oscillator (RFVCO) 250, thereby generating a signal ofthe frequency corresponding to the frequency difference of the signals;a phase comparator 236 for comparing an output of the offset mixer 235with a signal TXIF obtained by the adder 234 to detect the frequencydifference and the phase difference; a charge pump 237 for generating avoltage according to an output of the phase comparator 236; a loopfilter 238 for smoothing an output of the charge pump 237; a frequencydivider 239 for frequency-dividing an output of the TXVCO 240 togenerate a GSM transmission signal; and buffer circuits 241 a and 241 bfor converting a differential output to a single signal and outputtingthe single signal.

One of the buffer circuits 241 a and 241 b is a circuit for outputting asignal in the band of 850 to 900 MHz for GSM, and the other one is acircuit for outputting a signal in the band of 1800 to 1900 MHz for DCSand PCS.

On the chip of the RF IC 200 of the embodiment, a control circuit 260for controlling the whole chip, an RF synthesizer 261 and a loop filter263 constructing a PLL circuit for RF in cooperation with the RFoscillator (RFVCO) 250, an IF synthesizer 262 and a loop filter 264constructing a PLL circuit for IF in cooperation with theintermediate-frequency oscillator (IFVCO) 230, and a referenceoscillator (DCXO) 265 for generating a clock signal φref as a referencesignal of the synthesizers 261 and 262 are provided.

Although not shown, the synthesizers 261 and 262 have therein the fixedfrequency dividers 14 shown in FIG. 1 for frequency-dividing anoscillation signal of the reference oscillator 265. Thefrequency-divided clocks are used in the synthesizers 261 and 262. Eachof the synthesizers 261 and 262 includes a variable frequency dividerfor dividing oscillation signals of the VCOs 250 and 230, a phasecomparator, and a charge pump. Although not limited, in the embodiment,as the phase comparator 236 in the PLL circuit for transmission, ananalog phase comparator of high precision is used. As the phasecomparator in the RF synthesizer 261, a digital phase comparator havinghigh operation speed is used. Alternately, the phase comparator 236 maybe constructed by an analog phase comparator and a digital phasecomparator. In the beginning of operation, the digital phase comparatorof high speed is operated. After phases almost match each other, thedigital phase comparator is switched to the analog phase comparator ofhigh precision. In such a manner, pull-in operation in the beginning ofoperation of the PLL circuit can be performed at high speed and highprecision can be also obtained.

Since the reference oscillation signal φref is requested to have highfrequency precision, an external quartz resonator is connected to thereference oscillator 265. As the reference oscillation signal φref,frequency such as 26 MHz or 13 MHz is selected. A quartz resonator ofsuch a frequency is a general part and is available. In the embodimentof FIG. 6, the reference oscillation signal φref of 26 MHz isfrequency-divided into 1/65 by the fixed frequency divider, and theresultant is used as a clock of 400 kHz by the synthesizer 261.Similarly, the reference oscillation signal φref is frequency-divided to1/26 by the fixed frequency divider, and the resultant is used as aclock of 1 MHz by the synthesizer 262. In short, in the synthesizer 261,the clock of 400 kHz obtained by frequency-dividing the reference signalto 1/65 is used. In the synthesizer 262, the clock of 1 MHz obtained byfrequency-dividing the reference signal to 1/26 is used.

In the embodiment, the PLL circuit for transmission for performingfrequency conversion is constructed by the phase detector 236, chargepump 237, loop filter 238, oscillator (TXVCO) 240 for transmission, andoffset mixer 235. Although not shown in FIG. 6, a plurality of resistiveelements (R1 to R3) having different resistance values as resistiveelements of the loop filters 263, 264, and 238 and switch elements (SW1to SW3) for selection are provided. As the charge pumps in thesynthesizers 261 and 262 and the charge pump 237 of the transmissionsystem, as shown in FIG. 3, charge pumps having the plurality ofresistive elements R11 to R13 and R21 to R23) for switching an outputcurrent value and switch elements (SW11 to SW13 and SW21 to SW23) forselection are used.

The characteristic correcting circuit 18 of the embodiment is providedas a circuit common to three PLL circuits; the PLL circuit fortransmission, the PLL circuit for RF, and the PLL circuit for IF. Atpower-on, in the characteristic correcting circuit 18, time required forthe voltage V0 to reach V1 is measured and, on the basis of themeasurement, any of the resistor-selecting control signals CS1 to CS3 isdetermined.

The control circuit 260 in the RF IC of the embodiment has a controlregister in which a setting is made on the basis of a signal from thebaseband IC 300. Concretely, a clock signal CLK for synchronization, adata signal SDATA, and a load enable signal LEN as a control signal aresupplied from the baseband IC 300 to the RF IC 200. When the load enablesignal LEN is asserted to a valid level, the control circuit 260sequentially latches the data signals SDATA transmitted from thebaseband IC 300 synchronously with the clock signal CLK, sets them inthe control register and, according to the set data, generates a controlsignal for each of circuits in the IC. Although not limited, the datasignals SDATA are transmitted in series. The baseband IC 300 isconstructed by a microprocessor and the like. The data signal SDATAincludes a command sent from the baseband IC 300 to the RF IC 200.

In the multi-band radio communication system of the embodiment, forexample, by a command from the baseband IC 300, the control circuit 260changes the frequency φRF of the oscillation signal of the RF oscillator250 in accordance with a channel used at the time oftransmission/reception and changes the frequency of a signal to besupplied to the offset mixer 235 according to the mode which is the GSMmode or the DCS/PCS mode, thereby switching the transmission frequency.

The oscillation frequency of the RF oscillator (RFVCO) 250 is set to avalue which varies between the reception mode and the transmission mode.In the transmission mode, the oscillation frequency fRF of the RF IC(RFVCO) 250 is set to 3616 to 3716 MHz in the case of GSM850, 3840 to3980 MHz in the case of the GSM900, 3610 to 3730 MHz in the case of theDCS and, further, 3860 to 3980 MHz in the case of the PCS. Theoscillation frequency fPF is divided into ¼ in the case of GS and ½ inthe case of DCS and PCS, and the resultant signal is supplied to theoffset mixers 235 a and 235 b.

The offset mixer 235 a outputs a signal corresponding to the frequencydifference (fRF-fTX) between the oscillation signal φRF from the RFVCO250 and the oscillation signal φTX for transmission from the oscillator240 for transmission. The transmission PLL (TX-PLL) operates so that thefrequency of the difference signal coincides with the frequency of themodulation signal TXIF. In other words, the TXVCO 240 is controlled tooscillate at the frequency corresponding to the difference (offset)between the frequency (fRF/4 in the case of GSM or fRF/2 in the case ofDCS and PCS) of the oscillation signal φRF from the RFVCO 250 and thefrequency of the modulation signal TXIF. That is why the circuit iscalled an offset PLL.

FIG. 7 shows a configuration example of an RF IC realizing reduction inchip size by omitting the IFVCO by frequency-dividing the oscillationsignal φRF of the RFVCO 250 to generate a local signal of theintermediate frequency. The IF PLL circuit including the IFVCO, IFsynthesizer, and IF loop filter is not provided. Instead, an IFfrequency divider 266 for frequency-dividing an oscillation signal ofthe RFVCO to generate the signal φIF of the intermediate frequency isprovided. The RF PLL circuit takes the form of a fractional PLL in whichthe variable frequency divider in the RF synthesizer 261 is constructedby a circuit capable of frequency-dividing the oscillation signal φRF ofthe RFVCO 250 at a frequency division ratio given by an integer and afraction. The other configuration is similar to that of the system ofFIG. 6, so that detailed description will not be given.

In the embodiment, in the case of supplying the reference clock φref of26 MHz supplied from the DCXO 265 as it is to the RF synthesizer 261, itis desirable to use a third-order loop filter as shown in FIG. 5 as theloop filter of the RF PLL circuit. By using a third-order loop filter,noise in the high frequency domain can be reduced.

Although the invention achieved by the inventors herein has beenconcretely described above, obviously, the invention is not limited tothe foregoing embodiment but can be variously modified without departingfrom the gist. For example, in the embodiment, as resistors in the loopfilter and resistors in the charge pump, resistors whose resistancevalues are deviated from each other by 10% are prepared. The deviationamount among the resistors as components of the loop filter and thatamong the resistors as components of the charge pump do not have to bethe same. Different deviation amounts may be prepared.

Although the characteristic correcting circuit 18 is provided in theforegoing embodiment, the characteristic correcting circuit 18 may beomitted. In this case, the resistance value of the resistive element andthe capacitance value of the capacitative element in the loop filter aremeasured by a probe test or the like. By performing trimming using fusesand the like or by providing a register or a nonvolatile memory forholding trimming data, variations in resisters as components of the loopfilter and the charge pump can be corrected. In the case where means forholding trimming data is a register, trimming data can be supplied fromthe baseband circuit.

Further, in the foregoing embodiment, an output current of the chargepump is adjusted by switching the resistance values of the resistiveelements of the charge pump. Alternately, an output current of thecharge pump can be adjusted by providing a plurality of MOS transistorsof different sizes in parallel as the MOS transistors Q15 and Q25constructing the current mirror circuit in the charge pump in FIG. 3 andswitching the transistor to which current is passed among thetransistors.

Further, in the foregoing embodiment, the case of using the lag-leadfilter as the second-order loop filter has been described. The inventioncan be also applied to the case of using a filter of another form suchas a filter in which two first-order filters each constructed by asingle capacitative element and a single resistive element are arranged.

The case of applying the invention achieved by the inventors herein tothe RF PLL circuit, IF PLL circuit, and transmission PLL circuit builtin an RF IC as a component of a radio communication system in the fieldof use as the background of the invention has been described. However,the invention is not limited to the case but can be generally widelyused for a semiconductor integrated circuit with a PLL circuit.

1.-11. (canceled)
 12. A radio communication system, comprising: anantenna for transmitting and receiving signal electric waves; a switchwhich is coupled to the antenna for switching transmission andreception; a high-frequency power amplifier which is coupled to theswitch for amplifying a transmission signal; a radio-frequency IC whichis coupled to the switch and the high-frequency power amplifier fordemodulating a reception signal provided from the switch and modulatingthe transmission signal to be supplied to the high-frequency poweramplifier and switch; and a baseband circuit which is coupled to theradio-frequency IC for converting transmission data to I and Q signalsand controlling the radio-frequency IC, the radio-frequency IC formed ona semiconductor chip comprising: a voltage-controlled oscillator whichgenerates an oscillation signal; a variable frequency divider whichdivides a frequency of the oscillation signal; a reference oscillatorwhich generates a reference oscillation signal; a fixed frequencydivider which divides a frequency of the reference oscillation signal; aphase comparator which compares a phase of the oscillation signaldivided by the variable frequency divider with a phase of the referenceoscillation signal divided by the fixed frequency divider and generatesa signal in accordance with a phase difference thereof; a charge pumpcircuit which generates a charge current or discharge current accordingto the signal; and a loop filter which provides a voltage to thevoltage-controlled oscillator for controlling the frequency of theoscillation signal, the voltage being generated by smoothing the chargecurrent or discharge current by the loop filter; wherein a firstresistive element and a first capacitive element which is included inthe loop filter are formed on the semiconductor chip, wherein the chargepump circuit comprises a second resistive element which is constructedby a plurality of resistive elements having different resistance values,and the change in the charge current or discharge current is performedby selecting any of the plurality of resistive elements of the secondresistive element, and wherein the radio-frequency IC further comprisesa correcting circuit for changing the charge current or dischargecurrent in accordance with the selection of the resistive element.
 13. Aradio communication system according to claim 12, the radio-frequency ICfurther comprising a manufacture variation detector, the manufacturevariation detector including a current circuit that includes a thirdresistive element and outputs a current according to a resistance valueof the third resistive element, a second capacitive element charged bythe current circuit, and a voltage comparator which compares a chargevoltage of the second capacitive element with a reference potential,wherein selection of the second resistive element is determined inaccordance with a result of detection of a manufacture variation from achange in the charge voltage detected by the manufacture variationdetector.
 14. A radio communication system according to claim 13,wherein the loop filter is a second-order filter, and wherein the firstresistive element and the first capacitive element construct afirst-order filter circuit.